The project is concerned with understanding the organization and interactions of lipids and proteins in biological membranes. The proposal describes the use of appropriate bacterial strains and animal cell lines with mutations in membrane lipid biosynthesis for a series of chemical, physical, morphological, and biochemical experiments to delineate how fatty acid composition and sterol content determine the distribution and properties of phospholipids and the permeability and enzymatic activities of various types of membranes. Furthermore, the biochemical mechanisms by which cells conserve the properties of lipids important to structure and function will be investigated.